For example, in a projector for image display like a liquid crystal projector or a DLP (Trademark) projector, a high intensity discharge lamp (HID lamp) is used. In such a projector, by a dichroic prism etc., light is separated into the three primary colors of red, green, and blue, i.e., R, G, and B. A space modulation element provided for each color generates an image of each of the three primary colors, and optical paths thereof is combined by a dichroic prism etc., so as to display a color image.
In another known type of projector, light emitted from a light source is passed through a rotating filter (dynamic color filter) comprising a transmission color wheel having three primary colors (R, G, and B), thereby sequentially generating light rays of the three primary colors. In synchronization with the generated light rays, each of spatial modulation devices is controlled so as to sequentially generate an image of each of the three primary colors thereby displaying a color image.
As the light source for such a projector, a high intensity discharge lamp, such as a high-pressure mercury lamp, and a metal halide lamp, or a xenon discharge lamp, is used. However, in order to generate a high-definition image or a large area screen image in, for example, a movie theater in which good color-reproduction nature is required, a xenon discharge lamp in which light emission spectrum thereof is close to sunlight and a large electric power can be relatively easily realized, is mostly used.
FIG. 13 shows an example of schematic structure of a xenon discharge lamp. In a transparent outer enclosure (10) which is made from a quartz glass material etc. having high heat-resistance and which surrounds an electrical discharge space (Es) in which xenon gas is contained as a main component, a cathode electrode (E1) and an anode electrode (E2) which are made from a high heat-resistant and conductive material such as a tungsten etc. are provided so as to face each other, and, arc discharge is generated between these electrodes. During lighting, since electrons which are emitted from the cathode electrode (E1), and which reaches the anode electrode (E2), release their energy of motion as heat, the anode electrode (E2) produces heat remarkably. Therefore, the anode electrode (E2) needs to be larger than the cathode electrode (E1) in thickness and length by increasing the size thereof, so as to raise the heat dissipation efficiency. The cathode electrode (E1) and the anode electrode (E2) are connected to cathode side and the anode side mouthpieces (03, 05), through metallic foils (02, 04), which are made of such a molybdenum, respectively, for turning on electricity.
In case of a lamp shown in FIG. 13, in order to use efficiently the light emitted from the arc discharge area, a concave mirror which has a paraboloidal face or an ellipse face in a cross-sectional view is provided near the lamp, and the light is led to a following optical system, such as a light tunnel.
On the other hand, FIG. 14 shows a schematic view of an example of the structure of a xenon discharge lamp, in which an outer enclosure (70) surrounding an electrical discharge space (Es) also serves as a concave mirror. Such a xenon lamp is proposed in Japanese Laid Open Patent No. 09-161727. A cathode electrode (E1) is connected to a cathode mouthpiece (64) through electric conduction supports (61, 62, 63) made from a high heat-resistant and conductive material, such as a molybdenum plate etc., and the anode electrode (E2) is directly connected to an anode mouthpiece (65). Since no transparency is needed for an outer enclosure (70), it is made from a high heat-resistant ceramic material such as an alumina etc. However, a window (71) for extracting light is made from transparent material having high strength and high heat resistance such as sapphire etc. Metal covers (66, 67) for air sealing and protection are provided in joint areas of the outer enclosure (70), and the cathode mouthpiece (64) and the anode mouthpiece (65), respectively. Electric supply connection with the cathode mouthpiece (64) and the anode mouthpiece (65) is made through a conductive heat dissipation fin.
In a discharge lamp lighting apparatus for lighting the above-mentioned xenon discharge lamp, upon start-up of the lamp, while a voltage called a no-load open circuit voltage is impressed to the lamp, a high voltage is further impressed to the lamp by a starter, so that in its electrical discharge space, dielectric breakdown is generated, and a rush current with a suitable peak value is supplied thereto, thereby shifting to arc discharge so that lighting is started, and finally, and lighting in a stable steady state may be realized.
Usually, such a discharge lamp lighting apparatus has a converter which adjusts an output of an input power supply to the lamp discharge voltage so that a target lamp current required in order to realize a predetermined electric power applied to the lamp can be outputted. Moreover, the lamp voltage, i.e., the output voltage of the converter, is detected, and a certain unit determines the target lamp current, based on the information, for example, a value of the quotient which is obtained by dividing the target electric power by the detected voltage.
It is desirable that the life span of a light source lamp be long, not only in the above-mentioned projector but also all other uses, so temperature management during lighting is important for the long life span in a xenon discharge lamp. Although it may seem to be advantageous in respect of a long life span if the temperature of each part of the lamp is low, it is not true, that is, when the actual temperature is lower than the optimal temperature, the life span conversely becomes shorter.
In case of a xenon discharge lamp designed so that the temperature of a cathode electrode becomes low during lighting, since there is an effect that consumption (wear) of the cathode due to electric discharge can be controlled, the life span may become longer under condition of a continuous operation of lighting. However, since blackening of a lamp bulb is generated at time of starting of the lamp, there is a problem that the life span thereof becomes shorter than that of the discharge lamp designed so that the temperature of the cathode electrode does not become low during lighting, and this can be easily checked by performing blink lighting. Therefore, so far, lamps have been designed so as to optimize the temperature of a cathode electrode thereof, so that the life span thereof may become longer under the condition of a blink lighting operation.
On the other hand, in order to obtain high color-reproduction performance of a display image, it is important to adjust spectrum distribution of a light source lamp and the conversion form to the color sequential light flux using the above-mentioned dynamic color filter. In the case of the color wheel, it is possible to improve a color-reproduction performance or to carry out a desired color-reproduction performance, by setting up angle distribution of R, G, and B areas of the color wheel (depending on circumstances, W (white) may be added in addition to the R, G, B,)), that is, the rate of a period per rotation during which light transmits through each color area, according to the spectrum of the lamp.
For example, when a B component runs short, it is effective to enlarge the component transmission area thereof, so as to make the rate of a period during which light transmits through the B area, higher than the rates of the other color periods. However, in a DLP type projector, in order to obtain a desired color-reproduction performance by such a method, since the brightness for color of each pixel of a display image is controlled by the duty cycle ratio of each pixel operation of a space modulation element, there is a problem that it is difficult to carry out fine control of pixel tone in the color component whose rate of the period of light transmission is reduced.
In order to solve such a problem, for example, in Japanese Laid Open Patent (Tokuhyo) No. H08-505031, it is proposed that, in an image projection apparatus, a light source drive control unit which changes an output power of the light source, in synchronization with the color of an optical beam outputted from a color changing unit, is provided, so that light source intensity modulation is carried out.
Moreover, in Japanese Laid Open Patent No. H02-119005, a lighting apparatus in which the intensity of light emitted from a light source is adjusted according to the color of a filter area thereof, in synchronization with rotation of a rotation color wheel, is proposed. In addition, although this reference does not relate to a projector but an endoscope apparatus, it is not different from those disclosed in the other references, in that light source intensity modulation operation is added thereto. Therefore, both of them have the same problem which arises from the light source intensity modulation.
Thus, in order to solve above-mentioned problem, it is known that a light source intensity modulation which is synchronized with an operation of a conversion to color sequential light flux which uses a dynamic color filter, is useful. However, especially in the case of a xenon discharge lamp designed so that the temperature of a cathode electrode becomes low, there is a problem of the phenomenon of blackening as described above. In addition, when the lamp is turned on by a discharge lamp lighting apparatus which carries out an output current modulation for such light source intensity modulation, there is a problem that blackening of a lamp bulb tends to occur, and a solution of the problem was not found out.